For BeetlesMod, there are installation instructions for Counter-Strike Source that can be used to install L4D (since they're basically the same) But by the looks of the instructions, BeetlesMod is only for Windows servers. GameServers uses Linux.

On the other hand, SourceMod has both Windows and Linux (if you'd like to use it). They have lots of support on their forums and plugins are made by community members, so it is constantly evolving.

It's best not to have both SourceMod and Beetles, since they can conflict because of commands and such.